The present invention relates in general to joint prothesis systems for joints of the human body, and more particularly to a total shoulder replacement arthroplasty for replacement of confronting surfaces of the shoulder joint.
The shoulder joint, the largest joint in the upper extremity, functions as the fulcrum for all functional activities of the upper extremity, and also affords the greatest range of motion and planes of motion of any joint in the body. These unique characteristics of relatively high force transmission as well as the large range of motion afforded by the unique anatomical structure of this joint, impart certain difficult considerations in total shoulder prosthetic design. The design must allow for the shoulder to be the foundation joint for the upper extremity with its large range of motion as well as for the limited osseous framework available for fixation of the device. Additional considerations include the proximity of neural and vascular anatomic structures, the usual difficult surgical approach to the shoulder secondary to major muscular structures, and the relationship of the acromiom to the gleno-humeral articulation.
Much prior prosthetic replacement development has been directed at the hip and knee joints, the major weight bearing joints of the lower extremity. However, these joints have a much less complex and extensive range of motion and the anatomical considerations in these joints, especially the hip, are much less complicated than the shoulder. The surgical approach to the hip and the knee is generally considered less demanding than the surgical approaches to the shoulder joint.
Of the several surgial options available for the painful shoulder with limited or no motion, gleno-humeral replacement commonly accepted as total joint replacement for the shoulder has offered the best solution for problems of the entire shoulder joint to date. At present there are two basic categories of prosthesis available. These include constrained prosthesis and essentially unconstrained prosthesis. The major problems with the constrained prosthesis occur as in all constrained prosthesis, i.e., limited motion, mechanical failure, and loosening. In the unconstrained category the prosthesis usually available utilizes surface or flush glenoid components with fixation devices directed to the anatomical cancellous region of the scapulae. Fixation of the glenoid component has and continues to be considered a problem. Incomplete recovery of range of motion primarily in abduction and forward flexion has been encountered and appears to be related to overall prosthetic design. Violation of the medullary cavity of the humerous should be avoided, if possible. Another problem area in the present gleno-humeral replacement is proximal subluxation of the humerus with resulting humeral acromial impingement.
The shoulder is a complex articulation that involves the proximal humerus, the glenoid, the acromion, the clavicle and the scapulo-thoracic non-synovial articulation. The pathological process resulting in diseased and nonfunctional shoulders almost universally involve the articulation comprised of humerus, glenoid and acromion. Prior prosthetic designs with one exception, have not considered the acromial component.
Problems have been recognized in the presently available flush or surface mounted concave glenoid components in regard to dislocation of the humeral component and fixation. The flush mounted component offers a limited surface for attachment, thus necessarily limiting the surface area that may be provided for articulation with the humeral component. Recent design modifications to accommodate the acromial function would impart additional shear forces to the already tenuous glenoid component fixation and therefore contribute to the possibility of loosening. The surface mounted concave glenoid components now available offer no restriction to malposition referrable to retroversion or anteversion of the glenoid component.
Substantially all humeral component designs presently available utilize stemmed prosthesis. The experience with violation of the medullary cavity for placement of the stem and for fixation based on other joint prosthesis is well known. Violation of the medullary cavity increases blood loss and the amount of dissection required thereby increasing the likelihood of infection. An intramedullary stem significantly increases the surgical task, anesthesia time and blood loss should replacement of the humeral or stemmed prosthesis for any joint be necessary. The amount of methyl methacrylate and the surface exposed to the methacrylate is significantly increased with stem prosthesis thereby resulting in increased avascular or hypovascular tissue present in the wound which may be a contributing factor in late infection.
An object of the present invention, therefore, is the provision of a true total shoulder replacement arthroplasty system which encompasses replacement of the articular surface of the humerus with a convex surface replacement without a traditional stem and without violation of the medullary cavity by the prosthesis, and wherein the glenoid surface is replaced by a concave overmounted component, and the inferior aspect of the acromion is resurfaced with an acromial component for prevention of proximal subluxation.
A further object of the present invention is the provision of a shoulder replacement arthroplasty system as defined in the immediately preceding paragraph, wherein the acromial clavicular joint is replaced and mechanical advantage of a fulcrum is provided for abduction and forward flexion of the humerus.
Other objects, advantages, and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction with the accompany drawings illustrating a preferred embodiment of the invention.